Pathological activating mutations in the FLT3 kinase represent the most common genetic alteration in patients with acute myeloid leukemia (AML), occurring in approximately one third of cases. Recently, we performed translational studies that identified drug-resistant kinase domain mutations in FLT3-ITD at the time of relapse in AML patients treated with the FLT3 inhibitor, quizartinib (AC220), validating FLT3-ITD as a therapeutic target in human AML (N. Shah et al, Nature, 2012, 485, 260-263). To facilitate the rapid identification of potential therapeutic agents that can treat AML more effectively, a cancer with no newly approved efficacious treatments, we have created a novel fragment approach to discover kinase inhibitors and established a series of in vitro and in vivo assays to streamline the development of FLT3 inhibitors for AML. As a result, a highly potent FLT3 inhibitor clinical candidate, PMT-254, was identified. PMT-254 is the most potent, selective FLT3 inhibitor developed to date, displaying an IC50 of 0.009 nM for FLT-ITD transformed Ba/F3 cells. Further, PMT-254 achieves low nanomolar inhibition of quizartinib-resistant F691L, D835V, and D835Y mutants. Notably, all clinically-active investigational FLT3 inhibitors are much less potent and do not achieve a similar mutant activity profile. In human AML cell lines, PMT-254 achieves an IC50 of 0.15 nM in both MV4-11 and Molm-14 AML lines. In Molm-14 cells harboring an endogenous F691L or D835Y mutation, PMT-254 achieves a 0.226 nM or 2.2 nM IC50 value, respectively. PMT-254 does not impair the growth of parental Ba/F3 cells, even at a concentration of 10,000 nM. Screened against a kinase panel of 96 kinases, PMT-254 was found exclusively selective for FLT3 and FLT3 mutants. Clearly, PMT-254 is unlike any FLT3 inhibitor as manifested by its ultra-potency and ultra-selectivity, and therefore has extraordinary potential to transform treatment outcomes for patients with FLT3-ITD-driven AML. In MV4-11 xenograft studies, PMT-254 dosed PO 1.0 mg/kg shrunk tumors to undetectable levels in as little as four days (p <0.0001). Control tumors that reached a tumor volume of 1,200 mm3 were dosed at PO 1.0 and 0.3 mg/kg, which reduced tumor burden to undetectable levels in 11 days. Undetectable tumors from original treatment were allowed to rebound to a volume of 1,200 mm3 at which point treatment with PMT-254 commenced (PO 0.3 mg/kg), which again caused reduction in tumor volume to undetectable levels. Every mouse treated with PMT-254 was essentially cured of tumor burden and displayed no signs of toxicity even after receiving daily oral doses for over 40 consecutive days. In this proposal, we wish to further develop our FLT3 inhibitor by completing pilot formulation, PK/PD, and toxicity studies. This will acquire pivotal data necessary to justify completing an investigative new drug (IND) package. We have completed a preliminary 'proof of concept' data package, but specific facets to preclinical development are lacking that warrant additional pre-IND development. With the completion of this proposal, we will have a data package that will merit full IND development.